Organic Chemistry: Some Basic Principles and Techniques

Tetravalency of Carbon & Shapes of Organic Molecules

The electronic configuration of carbon in its ground state is $1s^2 2s^2 2p_x^1 2p_y^1$. Upon absorbing excitation energy, an electron promotes from the $2s$ to the vacant $2p_z$ orbital, establishing the tetravalent excited state: $1s^2 2s^1 2p_x^1 2p_y^1 2p_z^1$.

• Complete Structural Formula: Displays every single covalent bond explicitly as a dash ($ ext{C}- ext{H}, ext{C}- ext{C}$).
• Condensed Structural Formula: Omits individual bonds and groups identical repeating units using parentheses: e.g., Hexane as $ ext{CH}_3( ext{CH}_2)_4 ext{CH}_3$.
• Bond-Line Structural Representation: Carbon and hydrogen atoms are not written explicitly. Carbon atoms are implied at every vertex and line terminus, while hydrogen atoms are assumed to satisfy carbon's tetravalency. Heteroatoms ($ ext{O}, ext{N}, ext{S}, ext{Cl}$) and their attached hydrogens are written explicitly.
• Three-Dimensional (Wedge-and-Dash) Representation:
  • Solid line (—): Bond lying strictly in the plane of the paper.
  • Solid wedge (▲): Bond projecting forward toward the observer out of the paper plane.
  • Dashed wedge (▰/┄): Bond projecting backward away from the observer behind the plane.

• Acyclic (Open-Chain or Aliphatic): Saturated (alkanes) or unsaturated (alkenes, alkynes) straight or branched chains.
• Cyclic (Closed-Chain or Ring Compounds):
  • Homocyclic (Carbocyclic): Rings containing exclusively carbon atoms:
    • Alicyclic: Resemble aliphatic compounds in properties (e.g., Cyclopropane, Cyclohexane).
    • Aromatic: Follow Hückel's Rule [$(4n+2)\pi$ electrons, planar, cyclic, conjugated]. Divided into Benzenoid (e.g., Benzene, Naphthalene) and Non-Benzenoid (e.g., Tropylium cation, Azulene).
  • Heterocyclic: Rings containing at least one heteroatom ($ ext{O}, ext{N}, ext{S}$):
    • Alicyclic Heterocycles: Oxirane (ethylene oxide), Tetrahydrofuran (THF).
    • Aromatic Heterocycles: Pyridine ($ ext{C}_5 ext{H}_5 ext{N}$), Pyrrole ($ ext{C}_4 ext{H}_5 ext{N}$), Furan ($ ext{C}_4 ext{H}_4 ext{O}$), Thiophene ($ ext{C}_4 ext{H}_4 ext{S}$).

Systematic IUPAC Nomenclature

The International Union of Pure and Applied Chemistry (IUPAC) system constructs an organic name from five structural elements:

• Root Word: Denotes the number of carbon atoms in the longest continuous parent chain ($ ext{C}_1: ext{meth-}, ext{C}_2: ext{eth-}, ext{C}_3: ext{prop-}, ext{C}_4: ext{but-}, ext{C}_5: ext{pent-}, ext{C}_6: ext{hex-}, \dots$).
• Primary Suffix: Indicates the degree of saturation of the parent chain ($- ext{ane}$ for all single bonds, $- ext{ene}$ for double bonds, $- ext{yne}$ for triple bonds).
• Secondary Suffix: Identifies the principal functional group present in the molecule.
• Primary Prefix: Distinguishes cyclic compounds from acyclic ones (prefix cyclo-).
• Secondary Prefix: Identifies subordinate substituents (e.g., nitro $- ext{NO}_2$, halogen $-X$, alkoxy $- ext{OR}$, alkyl groups).

When an organic molecule contains multiple functional groups, the principal group determining the secondary suffix is chosen strictly according to the following IUPAC priority sequence:

All other functional groups of lower priority are treated as subordinate substituents and cited as prefixes.

1. Longest Continuous Chain Rule: Select the longest continuous carbon chain containing the principal functional group, maximum number of multiple bonds, and maximum substituents.
2. Lowest Locant Rule: Number the parent carbon chain from the end that assigns the lowest possible locant number to the principal functional group. If functional groups are identical, assign lowest numbers to multiple bonds (with double bonds preferred over triple bonds at equivalent positions), and then to substituents.
3. Lowest Set of Locants: When two numbering directions yield different sets of locants, compare them term by term; the correct sequence is the one that has the smaller number at the first point of difference.
4. Alphabetical Ordering: When writing the IUPAC name, list substituent prefixes in strictly alphabetical order (e.g., ethyl before methyl), ignoring multiplying prefixes like di-, tri-, tetra- (unless they are part of a complex substituent name like dimethylamino).

Isomerism in Organic Compounds

Two or more compounds having the same molecular formula but different chemical structures or spatial arrangements are called isomers. Isomerism is broadly divided into Structural Isomerism and Stereoisomerism.

• Chain Isomerism: Compounds with identical molecular formula but differing carbon skeletons: $$\text{Butane (} \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_3 \text{)} \quad \text{vs} \quad \text{2-Methylpropane (Isobutane, } \text{CH}_3\text{CH(CH}_3)\text{CH}_3 \text{)}$$
• Position Isomerism: Isomers that have the same carbon skeleton and functional group, but differ in the position of the functional group, multiple bond, or substituent: $$\text{Propan-1-ol (} \text{CH}_3\text{CH}_2\text{CH}_2\text{OH} \text{)} \quad \text{vs} \quad \text{Propan-2-ol (} \text{CH}_3\text{CH(OH)CH}_3 \text{)}$$ $$\text{But-1-ene} \quad \text{vs} \quad \text{But-2-ene}$$
• Functional Group Isomerism: Compounds possessing the same molecular formula but different functional groups, leading to entirely distinct chemical families: $$\text{C}_2\text{H}_6\text{O}: \quad \text{Ethanol (} \text{CH}_3\text{CH}_2\text{OH} \text{)} \quad \text{vs} \quad \text{Methoxymethane (Dimethyl ether, } \text{CH}_3\text{OCH}_3 \text{)}$$ $$\text{C}_3\text{H}_6\text{O}: \quad \text{Propanal (aldehyde)} \quad \text{vs} \quad \text{Propanone (ketone)}$$ $$\text{C}_3\text{H}_6\text{O}_2: \quad \text{Propanoic acid} \quad \text{vs} \quad \text{Methyl ethanoate (ester)}$$
• Metamerism: Arises due to the unequal distribution of carbon atoms (different alkyl chains) on either side of a polyvalent functional group ($-\text{O}-, -\text{S}-, -\text{NH}-, >\text{C}=\text{O}$): $$\text{Diethyl ether (} \text{C}_2\text{H}_5-\text{O}-\text{C}_2\text{H}_5 \text{)} \quad \text{vs} \quad \text{Methyl propyl ether (} \text{CH}_3-\text{O}-\text{C}_3\text{H}_7 \text{)}$$
• Tautomerism (Keto-Enol Tautomerism): A specialized dynamic structural isomerism wherein two functional isomers interconvert spontaneously via the rapid migration of an acidic hydrogen atom (proton) coupled with a $\pi$-bond shift. In keto-enol tautomerism: $$\text{CH}_3-\text{C}(=\text{O})-\text{CH}_3 \text{ (Keto form, 99.9%)} \rightleftharpoons \text{CH}_3-\text{C(OH)}=\text{CH}_2 \text{ (Enol form, 0.1%)}$$ The keto form is normally far more stable due to the exceptional bond energy of $\text{C}=\text{O}$ ($745\text{ kJ/mol}$) versus $\text{C}=\text{C}$ ($611\text{ kJ/mol}$). However, enol forms predominate when stabilized by intramolecular hydrogen bonding and resonance conjugation (e.g., $76\%$ enol in Acetylacetone $\text{CH}_3\text{COCH}_2\text{COCH}_3$, and $100\%$ enol in Phenol due to aromatic resonance energy).

Stereoisomers possess identical constitution and sequence of covalent bonds, but differ in the relative three-dimensional spatial orientation of their atoms.

• Geometrical Isomerism (*cis-trans* isomerism): Arises from restricted rotation around a double bond ($ ext{C}= ext{C}$) or rigid ring structure:
  • *cis*-isomer: Two identical or priority groups lie on the same side of the double bond.
  • *trans*-isomer: Two identical or priority groups lie on opposite sides of the double bond.
  • Example: *cis*-But-2-ene (dipole moment $\mu > 0$, higher bp) versus *trans*-But-2-ene ($\mu = 0$, higher mp due to symmetrical crystal packing).
• Optical Isomerism: Arises in molecules lacking any plane or center of symmetry, containing at least one asymmetric (chiral) carbon atom (a carbon bonded to four distinct substituents):
  • Enantiomers: Non-superimposable mirror image pairs (e.g., $d$- and $l$-lactic acid $ ext{CH}_3 ext{CH(OH)COOH}$). They rotate plane-polarized light by equal magnitudes but in opposite directions (dextrorotatory $[+]$ and laevorotatory $[-]$).
  • Racemic Mixture: An equimolar ($1:1$) mixture of two enantiomers, exhibiting zero net optical rotation due to external compensation.

Fundamental Concepts in Reaction Mechanisms

An organic reaction mechanism describes the detailed, step-by-step sequence of elementary bond-breaking and bond-forming events as reactants transform into products.

• Homolytic Fission (Symmetrical Cleavage): The shared electron pair is split equally between the two separating fragments, such that each atom retains one valence electron. This yields neutral, highly reactive species containing an unpaired electron, known as Free Radicals: $$\text{A}-\text{B} \xrightarrow{h\nu \text{ or heat}} \text{A}^\bullet + \text{B}^\bullet$$ Favored in non-polar solvents, gas phase, and in the presence of UV light or peroxides.
• Heterolytic Fission (Unsymmetrical Cleavage): The shared electron pair remains entirely with one of the fragments (the more electronegative atom): $$\text{C}-\text{Z} \to \text{C}^+ + :\text{Z}^- \quad (\text{Carbocation formation, if Z is more electronegative})$$ $$\text{C}-\text{M} \to \text{C}:^- + \text{M}^+ \quad (\text{Carbanion formation, if M is electropositive metal})$$ Favored in polar protic solvents.

• Electrophiles (Electron-Loving, $E^+$): Electron-deficient species that accept an electron pair from a substrate (Lewis acids).
  • Positively Charged: $ ext{H}^+, ext{NO}_2^+, ext{Cl}^+, ext{Br}^+, ext{R}_3 ext{C}^+, ext{H}_3 ext{O}^+$.
  • Neutral (Incomplete Octets): $ ext{BF}_3, ext{AlCl}_3, ext{FeCl}_3, ext{SO}_3, : ext{CCl}_2$ (dichlorocarbene).
• Nucleophiles (Nucleus-Loving, $Nu^-$): Electron-rich species possessing at least one unshared electron pair (Lewis bases).
  • Negatively Charged: $ ext{OH}^-, ext{OR}^-, ext{CN}^-, ext{Cl}^-, ext{Br}^-, ext{I}^-, ext{H}^-$.
  • Neutral Molecules with Lone Pairs: $ ext{H}_2\ddot{ ext{O}}, \ddot{ ext{N}} ext{H}_3, ext{R}\ddot{ ext{N}} ext{H}_2, ext{R}-\ddot{ ext{O}}- ext{H}$.
  • Ambident Nucleophiles: Species possessing two nucleophilic centers capable of attacking through either atom (e.g., cyanide $[: ext{C}\equiv ext{N}:]^-$ which can attack through carbon to form nitriles or nitrogen to form isonitriles; nitrite $[: ext{O}- ext{N}= ext{O}]^-$ forming alkyl nitrites or nitroalkanes).

Electronic Displacement Effects in Covalent Bonds

Electronic effects dictate the polarization, thermodynamic stability, and kinetic reactivity of organic molecules. They are categorized into Permanent Effects (operating in ground-state molecules) and Temporary Effects (operating only upon demand by an attacking reagent).

The permanent polarization of a $\sigma$-covalent bond arising from electronegativity differences between bonded atoms, transmitted along a carbon chain.

• Key Characteristics: Transmitted exclusively through $\sigma$-bonds; diminishes rapidly with distance and becomes virtually negligible beyond three carbon atoms.
• $-I$ Effect (Electron-Withdrawing Groups): Withdraw electron density more strongly than hydrogen: $$-\text{NO}_2 > -\text{CN} > -\text{COOH} > -\text{F} > -\text{Cl} > -\text{Br} > -\text{I} > -\text{OCH}_3 > -\text{C}_6\text{H}_5 > -\text{H}$$
• $+I$ Effect (Electron-Donating Groups): Release electron density relative to hydrogen: $$-\text{O}^- > -\text{COO}^- > -\text{C(CH}_3)_3 > -\text{CH(CH}_3)_2 > -\text{CH}_2\text{CH}_3 > -\text{CH}_3 > -\text{D} > -\text{H}$$
• Acidic Strength Application: Electron-withdrawing groups ($-I$) disperse negative charge on carboxylate anions, stabilizing them and increasing acidity: $$\text{CCl}_3\text{COOH} > \text{CHCl}_2\text{COOH} > \text{CH}_2\text{ClCOOH} > \text{CH}_3\text{COOH}$$

The instantaneous, complete transfer of a shared pair of $\pi$-electrons to one of the bonded atoms in an unsaturated compound, occurring strictly at the demand of an attacking reagent.

• $+E$ Effect: $\pi$-electrons transfer toward the atom to which the attacking reagent attaches (e.g., addition of $ ext{H}^+$ to alkenes).
• $-E$ Effect: $\pi$-electrons transfer away from the atom to which the reagent attaches (e.g., cyanide attack on a carbonyl group: $ ext{CN}^- + > ext{C}= ext{O} o > ext{C(CN)}- ext{O}^-$).

The polarity produced in a conjugated system through the interaction of two $\pi$-bonds or a $\pi$-bond and a lone pair on an adjacent atom.

• $+R / +M$ Effect: Groups donate electron pairs toward the conjugated system via resonance (ortho/para-directing activating groups): $$-\text{O}^- > -\text{NH}_2 > -\text{NHR} > -\text{OH} > -\text{OR} > -\text{NHCOCH}_3 > -\text{OCOR} > -\text{F} > -\text{Cl} > -\text{Br} > -\text{I}$$
• $-R / -M$ Effect: Groups withdraw electron density from the conjugated system via resonance (meta-directing deactivating groups): $$-\text{NO}_2 > -\text{CN} > -\text{SO}_3\text{H} > -\text{CHO} > >\text{C}=\text{O} > -\text{COOR} > -\text{COOH}$$
• Rules for Canonical Structure Stability:
  1. Structures with more covalent bonds and complete octets for all atoms are more stable.
  2. Structures with minimal charge separation are more stable than dipolar structures.
  3. Negative charge residing on the more electronegative atom ($ ext{O}, ext{N}$) and positive charge on the less electronegative atom ($ ext{C}$) maximizes stability.

The delocalization of $\sigma$-electrons of $ ext{C}- ext{H}$ bonds of an alkyl group directly attached to an atom with an unshared $p$-orbital (such as a carbocation, carbon radical, or $\pi$-electron system of an alkene).

• Because there is no formal bond between carbon and the proton in canonical forms, it is called no-bond resonance.
• Stability of Alkenes: Heat of hydrogenation is inversely proportional to alkene stability. Alkenes with more $lpha$-hydrogens have more hyperconjugative structures and are thermodynamically more stable (Saytzeff's Rule): $$\text{Tetramethylethylene (12 } \alpha\text{-H)} > \text{Trimethylethylene (9 } \alpha\text{-H)} > \text{trans-But-2-ene (6 } \alpha\text{-H)} > \text{But-1-ene (2 } \alpha\text{-H)}$$

Purification and Elemental Analysis

• Sublimation: Used for substances that pass directly from solid to vapor without melting (e.g., Camphor, Naphthalene, Benzoic acid).
• Crystallization: Relies on differential solubilities of compound and impurities in a suitably chosen solvent across temperatures.
• Simple Distillation: Purifies liquids boiling without decomposition that have large boiling point differences ($> 25 ext{ K}$) with impurities (e.g., Chloroform bp $334 ext{ K}$ and Aniline bp $457 ext{ K}$).
• Fractional Distillation: Uses fractionating columns (Vigreux) to separate miscible liquids whose boiling points differ by less than $25 ext{ K}$ (e.g., crude petroleum refining, acetone and methanol).
• Distillation under Reduced Pressure (Vacuum Distillation): Used to purify high-boiling liquids that decompose at or below their normal boiling points. Lowering the external pressure decreases the boiling point, enabling safe distillation (e.g., Glycerol boils with decomposition at $563 ext{ K}$ at $1 ext{ atm}$, but distills safely without decomposition at $453 ext{ K}$ under $12 ext{ mm Hg}$).
• Steam Distillation: Purifies liquids that are steam-volatile and completely immiscible with water (e.g., Aniline, Nitrobenzene, essential oils). At the distillation point: $P_{ ext{total}} = p_{ ext{organic}} + p_{ ext{water}} = 1 ext{ atm}$, allowing the mixture to boil below $373 ext{ K}$.
• Chromatography:
  • Adsorption Chromatography: Based on differential adsorption on stationary adsorbent ($ ext{Al}_2 ext{O}_3$ or silica gel). Includes Column Chromatography and Thin-Layer Chromatography (TLC). $$\text{Retention Factor } (R_f) = \frac{\text{Distance travelled by the substance from origin}}{\text{Distance travelled by the solvent front from origin}}$$
  • Partition Chromatography: Based on continuous differential partition between stationary and mobile liquid phases (e.g., Paper Chromatography).

Organic compounds are covalently bonded; to detect nitrogen, sulfur, and halogens, the compound is fused with molten sodium metal in a fusion tube to convert elements into water-soluble ionic sodium salts (Sodium Fusion Extract / Lassaigne's Filtrate):

• Detection of Nitrogen: Extract is boiled with $ ext{FeSO}_4$, acidified with concentrated $ ext{H}_2 ext{SO}_4$, and treated with $ ext{FeCl}_3$. Formation of a characteristic Prussian Blue precipitate or coloration confirms Nitrogen: $$6\text{NaCN} + \text{FeSO}_4 \to \text{Na}_4[\text{Fe(CN)}_6] + \text{Na}_2\text{SO}_4$$ $$\text{Fe}^{2+} \xrightarrow{\text{acid}} \text{Fe}^{3+}; \quad 3[\text{Fe(CN)}_6]^{4-} + 4\text{Fe}^{3+} \to \text{Fe}_4[\text{Fe(CN)}_6]_3 \downarrow \text{ (Prussian Blue)}$$ Note: If both N and S are present, blood-red $ ext{Fe(SCN)}_3$ forms instead.
• Detection of Sulfur:
  1. Sodium Nitroprusside Test: Extract + freshly prepared sodium nitroprusside yields an intense deep violet/purple coloration: $$\text{Na}_2\text{S} + \text{Na}_2[\text{Fe(CN)}_5\text{NO}] \to \text{Na}_4[\text{Fe(CN)}_5\text{NOS}] \text{ (Purple Complex)}$$
  2. Lead Acetate Test: Acidified with acetic acid + lead acetate yields black $ ext{PbS} \downarrow$.
• Detection of Halogens: Extract is acidified with concentrated $ ext{HNO}_3$ and boiled (to decompose and boil off any $ ext{HCN}$ and $ ext{H}_2 ext{S}$ gas which would otherwise precipitate with $ ext{AgNO}_3$), then cooled and treated with $ ext{AgNO}_3$:
  • Curdy white precipitate soluble in aqueous $ ext{NH}_4 ext{OH}$: Chlorine ($ ext{AgCl}$).
  • Pale yellow precipitate sparingly soluble in $ ext{NH}_4 ext{OH}$: Bromine ($ ext{AgBr}$).
  • Bright yellow precipitate completely insoluble in $ ext{NH}_4 ext{OH}$: Iodine ($ ext{AgI}$).

• Liebig's Combustion Method for Carbon and Hydrogen: $$\%C = \frac{12}{44} \times \frac{\text{Mass of CO}_2 \text{ formed (g)}}{\text{Mass of organic compound (g)}} \times 100$$ $$\%H = \frac{2}{18} \times \frac{\text{Mass of H}_2\text{O} \text{ formed (g)}}{\text{Mass of organic compound (g)}} \times 100$$
• Dumas Method for Nitrogen: Measured volume of $ ext{N}_2$ collected over $ ext{KOH}$ nitrometer reduced to STP: $$\%N = \frac{28}{22400} \times \frac{V_{\text{STP}} \text{ of N}_2 \text{ (mL)}}{\text{Mass of organic compound (g)}} \times 100$$
• Kjeldahl's Method for Nitrogen: Digestion with conc. $ ext{H}_2 ext{SO}_4$ converts N to $( ext{NH}_4)_2 ext{SO}_4$. Boiling with excess $ ext{NaOH}$ liberates $ ext{NH}_3$, absorbed into known volume of standard acid: $$\%N = \frac{1.4 \times N_1 \times V_1}{\text{Mass of organic compound (g)}}$$ (where $N_1, V_1$ represent normality and volume in mL of standard acid consumed by ammonia).
  Fatal Limitation: Kjeldahl's method is inapplicable to nitro ($-\text{NO}_2$), azo ($-\text{N}=\text{N}-$), or pyridine/quinoline nitrogen because these ring and oxidized nitrogen forms do not convert into ammonium sulfate under Kjeldahl digestion.
• Carius Method for Halogens & Sulfur: $$\%X = \frac{\text{Atomic Mass of } X}{\text{Molar Mass of Ag}X} \times \frac{\text{Mass of Ag}X \text{ (g)}}{\text{Mass of compound (g)}} \times 100$$ $$\%S = \frac{32}{233} \times \frac{\text{Mass of BaSO}_4 \text{ (g)}}{\text{Mass of compound (g)}} \times 100$$